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Optical Properties

Color and refractive index are the most important optical properties for fillers. Most filler applications require them to be colorless, or white, but traces of impurities can have significant deleterious effects on this.

The comparative refractive indices of fillers and polymers play a very important role in the appearance of composites, controlling whether filler effects are due to scattering or transmission of incident light. When the refractive index difference is large, then scattering dominates (the extreme example is for a filler or polymer powder in air, where the difference is great and the particles are seen by scattered light). As the refractive index difference reduces, so scattering decreases and transmission through the filler becomes more important, until, when they are identical, there is no scattering, just transmission. All of this has a profound effect on the appearance of the composite, both in terms of its transparency and color. When a filler is seen by scattered light, then its bulk color dominates and it can appear quite white; when it is by transmitted light, then small levels of impurities, such as iron or organics, can give very dark colors. The refractive indices of most common fillers and commodity polymers are similar, and thus, transmission rather than scattering dominates in composites. Surprisingly dark colors can sometimes be found from fillers which look quite white, when viewed in air. One way of checking for this, without having to make a composite, is to make a paste of the filler in oil with similar refractive index to the polymer.

The scattering power also depends on the filler particle size, going through a maximum at a certain size. This is usually quoted as being when the particle size is about one-third of the wavelength of the light; but this is an oversimplification.

The position of maximum scattering depends on the difference in refractive index between the filler and polymer, moving to larger particle sizes as the difference in refractive index between filler and polymer deceases. The one-third rule applies to situations where the refractive index difference is large, such as with titanium dioxide pigments. With most fillers, the size of maximum scattering is actually several times the wavelength of the light.

A further complication is that many fillers have more than one refractive index, and this can also give rise to complex and undesirable interference effects. Such effects are absent with amorphous fillers, such as glass and precipitated silica, and these are used when highest clarity is required.

In addition to their intrinsic optical effects, fillers can have an influence on the appearance of a composite for other reasons. One of these is gloss, which depends largely on the surface roughness of the composite. Larger filler particles or agglomerates of smaller ones can give rise to an irregular surface and thus low gloss, especially as measured at low angles, for example, 20°. Therefore, anything that can be done to minimize particle size and optimize dispersion will improve gloss. So, where this is important, one would select a finer grade of filler, use a dispersant, and set up processing parameters for high dispersion. Sometimes low gloss is desirable and large or poorly dispersed fillers may be added to deluster the surface. Sidings for houses or car dashboards are examples of surfaces that need to have low gloss.

 
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